Sacrifices had to be made if one wanted to maintain any semblance of peace in one's life. Even a healthy body has to cut off rotten parts in order to survive.
Their team was like that single body that was fighting against its enemy and had to sacrifice a limb in order to take down or at least survive the opponent that was stronger than them in terms of raw power and strength.
It was the only choice that had the potential to cut back their losses to the minimum. Both Zavier and Alissa understood that and were rushing to decide the actions to be taken in the worst-case scenario.
On the inside, Zavier didn't want to use such a method at all. He was sincerely praying with all his heart for Valerie so that she might be able to overcome the toxin present in her body.
If she really succumbed to it, then they would have to go ahead with their plan that would involve his sacrifice.